The treatment of water with bleach alone has been largely supplanted by the liquid chlorine process but the following details will be of use on meeting conditions for which liquid chlorine cannot be used and also for the preparation of the hypochlorite solution required in the chloramine process.
The essential features of a bleach installation are the solution or mixing tanks, storage tanks, piping system, discharge orifice or weir, and sludge drain.
Bleach is usually sent out by the manufacturers in sheet steel drums, 39 inches high and 291⁄2 inches in diameter, which contain about 14 cu. ft. of bleach and weigh approximately 750 pounds gross and 690 pounds net. It can be most economically purchased in car lots and if the consumption warrants this procedure storage should be provided for about 70 drums or rather more than one car load. According to Hooker[1] bleach loses 1 per cent of available chlorine per month in hot seasons and 0.3 per cent in cold ones so that it is advisable to carry as little stock as possible during hot weather. Hot weather also causes a further loss by accelerating the action of the bleach on the drum which rapidly disintegrates and cannot be handled. Bleach can often be purchased more cheaply in hot weather but such a policy is a short sighted one unless it is required for immediate use.
The general design of a hypochlorite plant is largely determined by the capacity but in all cases an effort should be made to avoid complicated details which may appear advantageous in the drafting office but do not stand up in actual practice. Many metals rapidly develop a protective coating on immersion in bleach solution but if this is removed by friction, rapid erosion ensues; bearing metallic surfaces should be reduced to a minimum.
Mixing Tanks. All tanks, whether mixing or storage, should be constructed of concrete and painted with two coats of asphalt. Experience has shown that wooden tanks are not suitable. The author has used pine, oak, and cypress tanks but all were rapidly leached by the hypochlorite and ultimately had to be lined with concrete.
There is a considerable variation in the concentration of bleach solution made in mixing tanks at various works. Some operators use about one gallon of water per pound of bleach and mix the two to a cream by wooden paddles, revolving on a central axis, for 1-2 hours; the paddles are then stopped and the cream run out into the storage tanks and diluted to the required strength by passing water through the mixing tank. There are two objections to this method: (1) the addition of small quantities of water to bleach tends to gelatinisation which may protect lumps from the further action of water and (2) a stratification of the solution occurs in the storage tank unless agitation is used. Gelatinisation causes loss of available chlorine and stratification causes irregular dosage unless corrected by agitation, which necessitates power. Other operators mix the bleach and water to the final concentration in the mixing tank and discharge the contents into the storage tank, the intermittent process being repeated until the storage tank is full. Gelatinisation is avoided by using a low original concentration and as all batches are of equal density no stratification is produced.
At Ottawa the bleach is crushed and, after weighing, dumped into a circular concrete tank provided with a hinged wooden lid. The stirring arrangement consists of a bronze shaft on which an aluminium impeller is fixed which revolves in an iron tube set slightly above the bottom of the tank (see [Fig. 1]). After the requisite amount of water has been added the motor connected to the bronze shaft is started and the mixture pumped for 15-20 minutes; without waiting for the sludge to settle the contents are discharged into the storage tank and the operation repeated until the tank is full. The piping between the mixing and storage tanks is of galvanised iron of generous dimension so as to compensate for incrustation. The pipes are straight and are provided with crosses at every change of direction to enable excessive incrustation to be removed. The valves should be made of hard rubber or special bronze; if brass valves are used they will probably require renewing every twelve months.
Fig. 1.—Mixing Tank for Bleach.